Each buffer has one major mode responsible for its principal editing or application commands. Programming-language modes work with source code, Dired with directories, EWW with Web pages, and Org with structured notes. Minor modes add optional behaviour without replacing the major mode.
The active mode determines which structures Emacs can navigate. In prose you might move by sentence; in Org by heading; in a list by item. Emacsvox speaks the result and can announce relevant state changes. To discover the mode’s commands, press C-h m, then C-h k followed by a key for more detail. Using Online Help explains these help commands.